The present invention generally relates to ceramic powders for forming coatings on components exposed to high temperatures, such as the hostile thermal environments of gas turbines. More particularly, this invention is directed to a ceramic powder having improved thermal insulation and erosion resistance properties for applications such as thermal barrier coatings (TBCs).
Improvements are continuously sought to increase the operating temperatures of gas turbines to achieve higher energy output and efficiencies. As a consequence of the higher operating temperatures, hot gas path (HGP) components within turbines are required to withstand the ever increasing temperatures. Often, hot gas path components are expected to operate at temperatures near their melting point. Consequently, complex cooling processes and improved materials are used to mitigate damage to the hot gas path components. In many instances, circumstances may necessitate further increasing the operating temperature of hot gas path components by depositing a thermal barrier coating on their exterior surfaces that are directly exposed to the hot gas path. The use of thermal barrier coatings (TBCs) on components such as combustors, high pressure turbine (HPT) blades, vanes and shrouds is increasing in commercial as well as military gas turbine engines. The thermal insulation provided by a TBC enables such components to survive higher operating temperatures, increases component durability, and improves engine reliability. TBCs are typically formed of a ceramic material and deposited on an environmentally-protective bond coat to form what is termed a TBC system.
Notable examples of ceramic materials for TBCs include zirconia partially or fully stabilized with yttria (yttrium oxide; Y2O3) or another oxide, such as magnesia, ceria, scandia and/or calcia, and optionally other oxides to reduce thermal conductivity. Binary yttria-stabilized zirconia (YSZ) is widely used as a TBC material because of its high temperature capability, low thermal conductivity, and relative ease of deposition. Zirconia is stabilized to inhibit a tetragonal to monoclinic crystal phase transformation at about 1000° C., which results in a volume change that can cause spallation. At room temperature, the more stable tetragonal phase is obtained and the monoclinic phase is minimized if zirconia is stabilized by at least about six weight percent yttria. A stabilizer (e.g., yttria) content of seventeen weight percent or more ensures a fully stable cubic crystal phase. The conventional practice has been to partially stabilize zirconia with six to eight weight percent yttria (6-8% YSZ) to obtain a TBC that is adherent and spallation-resistant when subjected to high temperature thermal cycling. Furthermore, partially stabilized YSZ (e.g., 6-8% YSZ) is known to be more erosion resistant than fully stabilized YSZ (e.g., 20% YSZ).
Various processes can be used to deposit TBC materials, including thermal spray processes such as air plasma spraying (APS), vacuum plasma spraying (VPS), low pressure plasma spraying (LPPS), and high velocity oxy-fuel (HVOF). TBCs employed in the highest temperature regions of gas turbine engines are often deposited by a physical vapor deposition (PVD), and particularly electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma/cathodic arc deposition, and all forms of melting and evaporation deposition processes (e.g., laser melting, etc.). TBCs formed by the various methods noted above generally have a lower thermal conductivity than a dense ceramic of the same composition as a result of the presence of microstructural defects and pores at and between grain boundaries of the TBC microstructure.
In order to improve TBC coatings, composite or clad powders have been developed which comprise more than one material wherein each material offers its own inherent material benefits. For example, the powder may be a metal-ceramic composite comprising a ductile metal matrix and a hard, wear resistant carbide phase. Alternatively, the powder may be a ceramic-polymer composite comprising either ceramic grains encapsulated in a polymer or a polymer encapsulated in a ceramic. The polymer material may be removed through oxidization of the resulting coating after consolidation. Removal of the polymer material yields an open porosity within the coating allowing the ceramic to be compliant in a turbine blade rub event. However, composite powders formed of more than one ceramic material are difficult to reliably form due to processing limitations.
In view of the above, it can be appreciated that improved coating materials are continuously sought in order to allow components to be capable of operating in higher temperature environments.